Thermosensitive recording materials are generally prepared by making dispersed fine particles of a leuco dye and fine particles of a color developer such as a phenolic compound respectively, then mixing these fine particles, adding thereto additives such as a binder, a sensitizer, a filler and a lubricant to obtain a liquid coating material, and applying the obtained coating material to paper, films, synthetic papers or the like. The recording materials produce color developments through chemical reactions that occur by melting one or both of the leuco dye and the color developer by heating to bring into contact with each other. To induce the color formation of such thermosensitive recording materials, for example, a thermal printer equipped with a thermal head is used. This thermosensitive recording method, as compared with other recording methods, is widely used in, for example, the fields of facsimiles, printers (computer output, calculators, etc.), recorders for medical measurements, automatic ticket machines and thermosensitive recording labels, because of its characteristics of: (1) no noise in recording; (2) no requirement to develop or fix an image; (3) maintenance-free; (4) a machine being relatively inexpensive; etc.
In recent years, in order to further expand applications of thermosensitive recording materials and to further increase efficiency, there have been increasing needs for high speed recording. Thus, it has been strongly desired to develop a thermosensitive recording material capable of sufficiently satisfying needs for high speed recording. In this case, a color developer needs to have a low melting point and less heat of fusion. Such properties, however, tend to cause deterioration of unrecorded parts (background fogging) of thermosensitive recording materials during production, use or storage. As a result, not only high whiteness but improvement in stability is strongly desired.
In general, a color developer having a phenolic hydroxy group has high ability to develop color. Among them, a large number of bisphenolic color developers including, for example, 2,2-bis(4-hydroxyphenylpropane) (bisphenol A) described in Patent Literature 1 and 4,4′-dihydroxydiphenylsulfone (bisphenol S) described in Patent Literature 2 have been reported because of their high coloring density. However, these color developers have disadvantages of less water and alcohol resistance, deterioration of background (background fogging) and the like.